This invention relates generally to molding, and more particularly to a modular mold for use in the manufacture and sale of molded objects.
The present invention has particular, but not exclusive, application in the field of molding, which is responsible for the production of many objects and components in numerous consumer and manufacturing markets. One particular application is for plastic injection molding, although other types of molding and casting fall within the scope of the present invention. Plastic injection molding machines have a fixture which receives a mold composed of two or more mold members or plates which are moved by the machine between open and closed positions. The mold members each contain mold cavities of unique geometric shapes, which partially define the shape of the molded objects produced by the mold. In the closed position, the mold plates come together, registering opposing mold cavities and defining one or more enclosed volumes having the shape of the object or component to be produced. The mold plates are secured in the closed position by the molding machine with sufficient force to remain sealed while resisting the expansive force of the mold material during charging of the mold. Liquefied molding material (e.g., plastic) is injected under pressure through a series of runner channels and a port into the enclosed volume, typically filling the available space in the volume. Thermal energy is removed so that the molding material solidifies within the enclosed volume. The mold plates are moved to the open position by the injection molding machine, and the molded object remains with one of the mold plates. An ejector device including ejection pins pushes the object and attached runners (formed by molding material in the runner channels) out of the one mold plate and the machine is ready to cycle again for the production of the next object. Molded objects are separated from runners either during ejection, or during a secondary, post molding operation, with degating being a commonly accepted term for this separation process. In instances of concurrent molding of multiple different objects, a sorting operation is also employed.
Plastic injection molding has enjoyed enormous commercial success because of its ability to produce large numbers of objects and components quickly and at low prices. Indeed, plastic injection molding may be the most prevalent method for the production of plastic objects. However, plastic injection molding has some drawbacks which limit its usefulness and can operate to prevent the introduction of certain types of products into the marketplace because of certain barriers to entry presented by plastic injection molding. More particularly, the mold which is used in the plastic injection molding machine is very costly to manufacture and maintain, requiring skilled artisans to produce and maintain. The cost savings previously mentioned are recognized only when a very great number of objects are manufactured. For products that will be sold in smaller numbers, or products which will be sold in numbers which are uncertain because of the uncertainty of commercial acceptance of the product, the cost of the mold is a large impediment to their production. The purchaser of molded parts is also faced with the dilemma of whether to spend the additional money to produce molds which are more efficient, i.e., as by having numerous cavities in a single mold for simultaneous production of many objects (parallel processing), or run the risk that if the product is needed in higher quantities than originally anticipated, an entirely new mold (or molds) will have to be purchased. This problem arises because the mold selected by the purchaser is strictly dedicated to production of one object (or group of objects) at one level of efficiency. Once constructed, the mold has essentially no flexibility in operation.
It is known that to reduce the financial risk associated with acquisition of an efficient production mold, it is possible to first produce, in a comparatively short time of fabrication, an inefficient, but low cost bridge mold, also known as a prototype mold. The bridge mold is capable of producing a small quantity of molded objects, and thus permit testing of the physical design, as well as market appeal of a molded object prior to committing to the typically larger financial investment and longer fabrication time associated with more efficient production molds. If molded objects produced by a bridge mold are found to be acceptable, the bridge mold may also be utilized to produce limited production quantities of molded objects, bridging the span of time required to fabricate an efficient production mold, and thus permit faster market availability of the molded objects than would be possible if only the final production mold were used for production.
In some instances, bridge molds may be produced by the same highly skilled artisan mold makers who are also employed to make production molds. The artisan mold makers use techniques for making the bridge molds that are similar to those used to fabricate production molds. In these instances of bridge mold fabrication, advantages of speed and economy are realized by compromising attributes of production molds. Such compromises typically include substitution of softer, more easily workable materials such as aluminum, as opposed to harder tool steel. Moreover, additive protective surface coatings for mold and cavity construction are not employed. Furthermore, the total number of mold cavities is typically limited to one for each object to be molded. And typically more primitive, less efficient methods of ejection, thermal regulation, degating and sorting are employed than utilized on production molds. However, even with these previously mentioned fabrication compromises, artisan mold makers are often able to produce complex molded objects which are nearly identical in shape, appearance and mechanical properties to those which will be produced by the final production mold.
Bridge molds produced by artisan mold makers have a number of disadvantages. For one, the cost and time required to fabricate a bridge mold is additive to the cost and time to fabricate the final efficient production mold. Therefore, molding projects utilizing bridge molding processes have higher total mold fabrication costs than molding projects that utilize only production molds. Furthermore, utilization of bridge molds extends the overall time of a molding project, as bridge molds are constructed as a first step, then following analysis and approval of the bridge mold produced prototype-molded objects, fabrication of a production mold may be commenced. While the costs of a bridge mold may be substantially less than a production mold, bridge molds fabricated by artisan mold makers are still quite expensive, owing to the typically high wages earned by artisan mold makers, and to the overall difficulty of hand crafting custom molds, even when employing the various shortcuts previously mentioned.
As an alternative to utilization of artisan mold makers to fabricate bridge molds in the traditional manner, several known systematic methods of mold design and fabrication may be used for the fabrication of bridge molds. In many instances these systematic mold fabrication methods may enable the fabrication of bridge molds faster and more economically than bridge molds fabricated by artisan mold makers. While being faster and less costly to fabricate, molds of these systematic processes contain all of the disadvantages of artisan-fabricated bridge molds. In addition to the disadvantages of the artisan fabricated molds, system constraints found in these systematic methods further limit molded object properties such as surface finish, part geometry and dimensional tolerances, and therefore often lack the capability to meet object design specifications.
Bridge molds, whether fabricated by artisan mold makers or by systematic processes, are subject to additional disadvantages which limit their usefulness. More particularly, these additional disadvantages are found when a bridge mold is utilized to meet interim production requirements, fulfilling market demands while a more efficient production mold is fabricated to replace the bridge mold. One of these disadvantages is that objects produced by an inefficient bridge mold have significantly greater per object production costs, which may offset and erode any profits realized by the earlier market entry facilitated by the bridge mold. Furthermore, the efficiency limitations of a bridge mold are also overall production capacity limitations. If the market success, and subsequent production demands of a molded object exceed the production capacity of the bridge mold, customer orders will go unfulfilled, which may result in customer dissatisfaction, and ultimately difficulty in retaining customers until greater production capacity is provided with the completed fabrication of a production mold. Being of temporary construction, bridge molds are also particularly susceptible to the effects of wear and damage, and as a result typically have short and unpredictable life spans, making them unreliable for production molding, even on an interim basis, as the bridge mold may fail before a production mold is fabricated. The cost risks associated with insufficient production capacity and unreliability of a bridge mold are magnified when the molded objects produced by the mold are a unique component part of product containing many parts. The delivery failure of the one unique part will interrupt the delivery of the entire dependant product, and may result in lost sales of much greater scale than the costs of the individual molded object.
Production molds may be designed to provide different levels of capacity and production efficiency, but these differing levels of capacity and efficiency have associated costs, which typically increase as the level of capacity and efficiency of the mold design is increased. Therefore, design and investment decisions of production molds require an assessment of the total molded object production requirements in order to select the most appropriate level of capacity and efficiency. As previously mentioned, fabrication of bridge molds prior to the design and fabrication of production molds enables a limited assessment of potential market acceptance and demand for molded objects. While production predictions based on market assessments from these bridge molded objects are useful, their accuracy and reliability are limited, as any prediction of future events is speculative. Furthermore, market demand for a particular molded object tends to change throughout the life cycle of the object, typically first growing as the market adopts the object, then declining as its life matures. Therefore, even if an accurate prediction of the overall demand for molded objects were possible, such predictions would still be inaccurate during various segments of the object's life cycle, and as such it is essentially impossible to make a single mold design and investment decision that is optimal for all phases of the molded objects life cycle.
What is needed is a modular mold and modular method of molding capable of providing rapid and economical fabrication of bridge molds that can then be rapidly and economically upgraded and transformed into an efficient production mold, and also capable of meeting variable capacity and efficiency levels.
It is known to provide some additional flexibility in mold making by constructing a mold which is modular. Instead of mold plates that are each monolithic, the plates are formed as frames which are capable of receiving several mold inserts. The mold inserts contain the mold cavities which mate with the mold cavities of corresponding mold inserts to define the mold volumes in the shape of the object or objects to be produced. The mold so configured may produce many of the same object or produce several different objects in a single mold cycle. Using a modular approach, much less material is required to form a mold insert than would ordinarily be required to form the entire mold plate with a cavity. The frame is generic and can receive different arrangements of mold inserts, and so the overall cost of producing a mold can be reduced. However, it is believed that the full potential of modular molds has not been exploited because of marketing methods which are still focused on single use molds.
Morever, modular molds suffer to a greater degree from a problem which is generally present in plastic injection molding. Although generally considered being an efficient manufacturing process, one of the primary impediments to molding efficiency is the time in which the mold is at rest after the plastic is injected into the mold, waiting for the plastic to solidify. The solidification time is a function of the heat transfer rate out of the mold volume after hot molding material is injected into the mold. The use of mold inserts may exacerbate this problem because there is insufficient contact with adjacent components of the mold to produce the most ideal conductive heat transfer. As a result, the cycle time of the injection molding machine may be increased with a modular mold. Some attempts to resolve this problem have been made, such as by having the mold insert contain its own liquid coolant circulation loop connected to the coolant system of the injection molding machine. However, this requires that the mold insert be larger, increasing its costs and reducing its flexibility of positioning within the mold plate. The fluid connections to the mold insert required every time the mold is reconfigured are complex and a source of manufacturing delay, and mold configurations and designs are limited by the need to provide for such fluid connections. Still further, steel, the common material used in mold manufacture, does not have the most ideal heat transfer characteristics. In addition to transferring heat out of the mold at a lower rate, the heat transfer is not uniform, so that there may be hot and cold spots in the mold. It is known to use aluminum, which has better heat transfer characteristics, but aluminum is less resistant to wear and subject to greater thermal expansion and contraction within the mold.
Another issue associated with existing injection molding molds and process relates to the reconditioning of molds. Over time, the molds (regardless of the type of material from which they are made) will wear to the point that reconditioning is required. Conventionally, skilled craftsmen are employed to perform this task. Reconditioning involves cutting down the mold to remove damage or wear, following by reforming of the cavity and runner channels leading to the cavity. The reconditioning causes the height of the mold to change, which can be particularly problematic if attempted for modular molds where the height and location of the upper surface of the mold inserts must remain the same for all mold cavities to seal.
Still further, the modularity of the mold inserts is limited by the modularity of the runner channels delivering liquefied molding material to the inserts. Conventionally, the runner channels have been as dedicated to a single use as the molds themselves. Providing a modular mold using mold inserts still requires that the liquefied molding material be delivered in some manner to the mold inserts. Presently, these runner channels are dedicated to a particular mold insert, making it difficult to reconfigure the mold. Mold inserts conventionally must be made of the same material so that they have the same thermal expansion in use. Even if made of the same material, mold inserts are more difficult than one piece molds to register with mating mold inserts to form a sealed mold enclosure volume because of problems with accurately positioning removable mold inserts in the mold frame.